A quiet little spot where Rod Mollise shares his adventures and misadventures...

Sunday, March 19, 2017

Issue #535: The Final Piece of the Puzzle

In our pre-spring observing season drive to get novices (and
maybe even a few not-so-novices) set up with a rig for deep sky imaging, we’ve
addressed mounts, telescopes, and, last week, auto-guiding setups. This Sunday
we’ll finish with suggestions for a low-cost camera. I’ve talked about imaging
cameras with y’all fairly recently, but the difference is that this time I’ll
try as hard as I can to keep the cost as low as possible.

So, you need a camera and a few accessories. Where do you
start? The first question to answer is, “Do I want color?” While a monochrome
CCD/CMOS astronomical camera can take color images by exposing successive
frames through three or more colored filters, it’s not something you want to
face when you are just getting off the ground in imaging. Unless you enter the
ranks of the hard-core someday, you may never
want to face it. In the beginning you will find just processing a “one-shot”
color image enough of a challenge. Properly calibrating and combining three +
separate frames into a color frame and then stacking and processing a bunch of
those? Uh-uh.

So, it’s a color camera, a one-shot color camera, you want.
How does one work? A color camera is different from a monochrome camera in that
red, green, and blue color filters are built into the sensor chip. Software,
either in the camera or in an image processing program, automatically combines
the R, G, and B to produce a full color image. That is usually transparent to
the user—with a digital single lens reflex (DSLR), anyway. You take a picture,
you see a color image, end of story.

Some astrophotographers say a monochrome camera can produce
visibly higher resolution images because it doesn’t waste pixels on the
production of a color image. In truth, in the beginning at least, and
especially on deep sky objects, you won’t notice any difference.

The next question is “CCD or CMOS?” That is not much of a
question today. Unless you are interested in some special applications, mostly
having to do with obtaining scientific data, there is no reason to choose a CCD
chip over a CMOS chip. Today, the formerly preferred CCD has lost ground to
CMOS sensors even for use in “astronomical” cameras. CMOS chips are now very
sensitive and very low in noise. At any rate, almost all cameras in our price
range, which I am topping out at 450 dollars, have CMOS chips, so the choice
has already been made for you.

What a ZWO ASI120MC can shoot...

Next up, cooling. “Does a camera for taking long-exposure
images need to have its sensor chilled to reduce thermal noise?” Today, probably
not. With some camera/chip combos, an internal fan, at least, can be helpful to
reduce the false stars of thermal noise, but the low-noise characteristics of
today’s sensors usually means subtracting a dark frame is enough to deal with
thermal noise.

And the Final Jeopardy Question… “Astro cam or DSLR?” There are some interesting low cost
astronomical cameras coming on line, like those from China’s ZWO, and I’ve
actually taken credible deep sky image with one of their 1/3-inch cameras that
cost a measly 200 dollars. However, I think for most of us a DSLR is just a
much more sensible choice. A much more sensible choice.

Why is a DSLR better? There are several reasons, but there
is one real big one: when you’re not
taking pictures of the night sky, you can be wowing everybody at your mother-in-law
Margie’s birthday party with your snapshotting skills. There’s also that big
elephant in the living room. Like many wannabe astrophotographers, a few nights
wrestling with camera and scope may convince you you are actually more of a visual observer. If that be the case,
you can still get years of use and enjoyment out of the DSLR, even if you never
take another astrophoto with it.

Another big plus (for astro imaging) of the DSLR? Their
relatively big chips. A less than 500 dollar camera will have an APS-C size
chip. Lower cost astro-cams tend to have small chips that restrict your field
of view, focal length for focal length, and also tend to make guiding more
critical.

Finally, while I control my DSLRs with a program running on
a laptop (“tether them,” as we say in the photography business), which makes
focusing and framing much easier, you don’t have
to do that. You don’t have to have a computer out in the field when you are
taking pictures. You can do just as we did in the SLR days: telescope, mount, camera. You will, as in those
SLR days, need a remote camera release (an intervalometer, preferably), but
that is it.

OK, so which DSLR?
The safe thing to say is still “Canon.” In some ways they still lead the pack
in astrophotography. The Canons are remarkably low in noise over long exposures,
and are easy to use in the field with a laptop if you choose to do that. Things
are changing now, but until recently camera control software (like Nebulosity)
was unheard of for other brands.

SCT Prime Focus Adapter

There’s also Canon’s longstanding involvement in our game. While
Nikon and, now, Pentax are coming on strong for astrophotography, until the
last couple of years only Canon acknowledged people were actually using their
cameras for astronomical imaging and produced cameras with astronomy in mind.

Canon is a safe choice, in my opinion, but which one of
their many DSLRs? If you are buying new and must keep the price tag low, the Rebel T6, which is available for about 450
dollars, is a remarkable value. Not
only do you get a DSLR that will perform well for astro-imaging or anything
else, you get a pretty good (zoom) kit lens for use in wide-field astrophotography
or at Margie’s above mentioned b-day party.

Just don’t want a Canon for whatever reason? The equivalent
Nikon is the D3300, which is even less expensive than the Rebel. And it can
perform very well for astronomical imaging. BUT… Computer control options for
this camera are (very) limited—it is not supported by the major Nikon
astrophotography program, BackyardNikon—so if you want to tether camera to
computer, a Canon is a far better choice.

How about buying a used camera? Is that a good idea? That
depends. A fairly recent camera or seldom used older camera can push prices
even lower. A perfectly serviceable older Rebel, like a 450D, for example, goes
for 150 or fewer dollars with a kit lens and a few accessories. Be careful
here, though. While the Rebels, Canon’s introductory DSLRs, and Nikon’s
comparable models are well-made, they are not professional grade cameras and
won’t stand up to real abuse. So, when considering an inexpensive camera it’s
best to limit yourself to one that’s for sale locally so you can examine it in
person and make sure it’s fully functional.

Accessories

Prime Focus Adapter

Prime focus adapter (1.25-inch)...

Once you’ve got a camera, of course you’ll need accessories.
You always need accessories in
astronomy, you know that! First off, you
will need a prime focus adapter in order to connect camera to telescope.
“Which” depends on your scope style. SCT prime focus adapters screw onto the
SCT’s rear port. Those for other telescope designs, like refractors, typically
have 1.25-inch or 2-inch nosepieces and slide into the scope’s focuser. I like
the 2-inch models, not because you have to worry about vignetting or something like
that with an APS-C size sensor, but because they allow me to dispense with a
1.25 – 2-inch eyepiece adapter and seem to provide a more secure mounting
arrangement.

T-ring

You’ll also need a t-adapter for your camera, aka a
“t-ring.” This is a, yes, ring shaped adapter with T-threads on one end to
screw onto the prime focus adapter, and a lens mount for your particular camera
on the other end. These two things in hand, you can remove the camera’s lens,
mount the combo of T-ring/prime focus adapter in its place, and then mount the
camera on your scope by inserting everything into the focuser or screwing the
prime focus adapter onto the rear port of an SCT.

Intervalometer

As you may know, DSLRs, most of them anyway, and certainly
all the Canons, can’t expose for more than 30-seconds without the addition of a
remote shutter release. Even if your camera could
expose for longer without a remote, you’d still want one as it allows you to
trip the shutter without bumping the scope and causing trailed stars.

T-ring

An intervalometer is a remote shutter release, but it’s also
much more. Not only will one of these (usually) wired controls allow you to trip
the shutter from a distance and expose for as long as you like, it will allow
you to shoot sequences of images. Say
30 3-minute exposures, which is exactly what we want to do. An intervalometer
allows you to do many of the things a tethered computer would allow you to do,
but without the computer. How much? A Vellois
about 50 bucks and a genuine Canon is about three times that. Guess which one I’d choose?

Memory Card

If you’re not using a tethered PC, you’ll have to have a
memory card, digital "film" on which to store your images. An SD card (used by almost all
DSLRs, now) with at least 64gb capacity is my recommendation—you’d be surprised
how much space an evening’s images can take up. Get a good, decently fast card.
I like the Sandisk ones. About 40-bucks.

Battery

If you’re going to use a battery, make sure you keep an
extra, or, better, two extras in your
gadget bag. During long exposures, the camera is drawing current from the battery
continuously, and you’re unlikely to get a full evening out of one cell,
especially on cold nights. There are lots of third party batteries available,
but I have had noticeably better performance out of genuine Canon, so that’s
what I recommend here, the real deal, for a change.

Power Supply

Yes, batteries are a problem during astrophotography, so
don’t use one, or use a real big one. Hop on over to Amazon and buy yourself
either a 12vdc or 120vac power brick for your Canon (or whatever). I do most of
my shooting at locations with mains power, so I prefer the AC option. The DC
supplies have cigarette lighter plugs that will plug right into your jumpstart
battery pack.

What do you plug one of these things into on the camera end?
These power supplies have little plastic (wired) widgets that take the place of
the normal battery in the battery compartment and supply power to the camera
that way. I’ve found one of the inexpensive—less than 15-dollars—units on
Amazon to work just fine, but Canon will sell you one for considerably more if
you like.

Anything else? Well, a few things, maybe. If you are new to
DSLR photography, you probably want a camera bag, a gadget bag, to keep camera
and lenses and, well, gadgets,
together. A nice piggyback bracket so you can mount DSLR and lens on your
telescope tube is a nice addition and you may find you like doing wide-field
shots from dark locations. A lenspen is good to keep your lens’ surface
pristine. A broadband light pollution filter can be helpful if, like me, you do
some of your imaging from an at least somewhat light-polluted backyard. And
that is really more than enough to get you started.

You’ve now got all the pieces to the complicated astrophotography
puzzle, but how the heck do you put them together? We’ll talk about that, about
getting started with all this stuff, next week.

Addendum: How good can a VX be?

Auto-guiding wise, that is. Some of you considering a
Celestron Advanced VX mount (or the similar mounts on the market today) have expressed
grave concern about my statement last week that 2” (arc seconds) of RMS guiding
error is about what you should expect of this group without some fine-tuning
(of PHD’s Brain Icon settings, I mean).

Anyhow, while 2” is perfectly suitable for some image
scale/camera pixel combos, naturally it would be nice to do a bit better with this
inexpensive and highly portable GEM. So, I set about the other night to see how
much and how easily I could tweak the VX.

Surprise! I really didn’t have to do much tweaking at all to
get this modest mount’s RMS guiding error down. I did do a decent polar
alignment, and I did spend some time carefully balancing the scope (east heavy
with a little declination bias as
well). As for the settings, I backed off on a couple of them. Cutting
aggressiveness in half and reducing hysteresis as well. Oh, and, conversely, I increased Max Duration both for RA and
declination.

The result? Despite OK but hardly great seeing, my errors
were immediately halved with me getting just under 1” of RMS error most of the
time. Even when my target got low in the sky, and seeing began to deteriorate,
the error was just over 1”, easily good enough to yield round stars with an
80mm f/6.9 despite the fairly small (1/2-inch) sensor of the camera I was testing.

While I warned you not to start chasing lower and lower
numbers with these GP/CG5 clone mounts merely for the sake of lower numbers,
given the small amount of effort involved in this substantial improvement, the
few minutes I spent was well worth it.

The other take-aways? People naturally worry about their
guide-software settings, but what makes one of the very largest differences? Seeing.
Without good seeing you will not see great guiding, so don’t start messing with
your settings on an unsteady night. Oh, and good polar alignment is important
for good guiding as well. Having to continually chase alignment-caused drift
just muddies the water and makes guiding more difficult to get right. Finally,
with this class of mounts, correct balance is just as important as polar
alignment and seeing. If you want 1” or less guiding errors, you’ll likely need
to rebalance if you move to a radically different part of the sky—cross the
Meridian, etc.